The public power grid supplies alternating electrical currents to the consumers. The power grid must comply with predetermined requirements such as target frequencies of the alternating current distributed by the power grid. The frequency of the alternating current within the power grid can be different for different states or countries. For instance, in Germany the basic frequency of the voltage provided by the public power grid is 50 Hz, whereas in the USA it is 60 Hz. The equipment connected to the power grid is normally unable to operate under significant deviations from the target frequency. For example, transmission system operators in Europe contract equipment operators to commence stabilization measures as soon as the grid frequency of the power grid deviates by more than 0.01 Hz from the target frequency of 50 Hz. Accordingly, it is necessary to provide grid services to the power grid where stabilization measures are required. These stabilization measures either add power to the grid or remove power from the grid in order to stabilize the frequency of the power grid. In a conventional power system, this is achieved by adjusting the power output of power generation units operating below their maximum power output capability, or by deploying various forms of energy storage devices including pumped hydro power plants, accumulators/batteries or flywheels. Further, it has been proposed to control the heating of residential homes that consume power.
Electrical storage devices, such as capacitors, store energy by separation of electrical charges. Electrochemical storage devices, in particular batteries or accumulators, additionally store energy by separation of materials with different electrode potentials. Mechanical storage devices store energy by elevation of mass in a gravitational field, mechanical compression or, in the case of a spring, mechanical extension, or rotational energy (so called Flywheel). All energy storage device comprise a maximum storage capacity or a maximum state of charge SOCmax, either determined by the characteristics of the underlying process and the materials used therein, or determined by characteristics of the implementation, such as the type and quality of membranes in batteries. The current state of charge of an electrochemical storage device is the amount of energy which can be drawn from the battery and can be indicated as a percentage of the maximum state of charge of the respective battery. The maximum state of charge of any battery deteriorates with use, i.e. charging and discharging, of the battery. The end of life of a battery is thus frequently defined in terms of a minimal maximum state of charge, although this end-of-life condition can also be chosen in terms of other performance characteristics of the battery, such as the internal resistance. In general, charging a battery whose state of charge is close to the maximum state of charge will bring about more deterioration of the battery than the same charging at lower states of charge. Furthermore, the maximum power output of a battery at a state of charge which is close to the minimum state of charge is lower than the maximum power output at higher states of charge, and will at a certain state of charge be insufficient to satisfy the requirements of the application in which the battery is deployed. Similar rules govern all other forms of energy storage. Therefore, for any combination of an energy storage device and an application, a lower state of charge limit SOC-L and an upper state of charge limit SOC-H is defined. If the charge stored in the energy storage device exceeds the upper state of charge limit SOC-H of the respective energy storage device, the lifespan or possible operation time of the energy storage device is reduced to an commercially unacceptable extent. Further, if the charge drops beneath the lower state of charge limit SOC-L of the respective energy storage device, the storage device will not be able to satisfy the requirements of the application in which the storage device is deployed. Accordingly, an energy storage device should be operated in a state of charge range between the upper state of charge limit SOC-H and the lower state of charge limit SOC-L.
An energy storage device ESD is normally controlled by an energy resource controller ERC which can be connected via a communication device or communication interface and a communication link to a remote, central or distributed control unit of the power supply grid. If the state of charge of the energy storage device controlled by the energy resource controller exceeds the upper state of charge limit SOC-H the energy resource controller can send a signal to the control unit that excessive electrical energy is locally available and can be provided to other users or facilities connected to the power grid. The excessive electrical energy of the energy storage device can then be provided to other consumers or storage devices to reduce the state of charge of the respective energy storage device such that it falls beneath the upper state of charge limit SOC-H of the energy storage device. However, in such a conventional system, there can be a significant time delay until the exceeding electrical power in the energy storage device can be supplied to other consumers. Because of this time delay, it can happen that the state of charge SOC of the energy storage device still increases during the delay time and can exceed even the maximum state of charge SOCmax of the respective energy storage device. In this case, the energy storage device can be severely damaged or even destroyed. Moreover, as long as the state of charge of the energy storage device exceeds the upper state of charge limit SOC-H, the energy storage device is in a state where its operation lifespan is diminished by additional charging. It is also possible in a conventional system that the communication link connecting the communication interface of the energy resource controller with the control unit is interrupted or its bandwidth is limited so that no reaction at all takes place and the state of charge SOC of the energy storage devices can increase until the energy storage device is severely damaged. In a worst-case scenario where additional security measures are not taken, a battery mistreated in such a fashion could even start to overheat thus causing severe security problems in the surrounding of the energy storage device.
Accordingly, there is a need for a method and apparatus for providing a grid service to a power grid which overcomes the above-mentioned problems and which in particular increases the overall operation lifetime of energy storage devices used by said power system.